• Journal of Mechanical Science and Technology
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• v.20 no.10
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• pp.1680-1690
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• 2006

As a promising and novel manufacturing technology, laser aided direct metal deposition (DMD) process produces near-net-shape functional metal parts directly from 3-D CAD models by repeating laser cladding layer by layer. The key of the build-up mechanism is the effective control of powder delivery and laser power to be irradiated into the melt-pool. A feedback control system using two sets of optical height sensors is designed for monitoring the melt-pool and real-time control of deposition dimension. With the feedback height control system, the dimensions of part can be controlled within designed tolerance maintaining real time control of each layer thickness. Clad nugget shapes reveal that the feedback control can affect the nugget size and morphology of microstructure. The pore/void level can be controlled by utilizing pulsed-mode laser and proper design of deposition tool-path. With the present configuration of the control system, it is believed that more innovation of the DMD process is possible to the deposition of layers in 3-D slice.

• Journal of Biosystems Engineering
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• v.30 no.5 s.112
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• pp.268-273
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• 2005

This study was conducted to investigate the specificities of four fulcrum types for geo-statistical analysis of elevation within rice paddy field. In Korea, the spaces between inter-rows and between hills for rice transplanting are 30cm and 11cm to 14cm, respectively. So, the size and shape of fulcrum for field elevation measurement should be considered according to the inter-row and the hill spaces. Four kinds of fulcrum were chosen such as round-shape with 2.5cm diameter, circular-shape with 10cm diameter, 10cm (one third of inter-row space) by 24cm (double of hill space) rectangular-shape, and 20cm (two-thirds of inter-row space) by 24cm rectangular-shape. The resulting descriptive statistics couldn't determine the best fulcrum type to measure the rice paddy field elevation. But the results of geo-statistical analysis could determine the best fulcrum type. In the case of 10cm by 24cm rectangular-shape fulcrum, Nugget and range, meaning measurement error and/or noise, and limit of spatial connection, respectively, were minimum; Q value meaning weight of spatial structure and $r^2$ value were minimum, and residual sum of squares was minimum. It means that 10cm by 24 cm rectangular-shape fulcrum could best describe the rice paddy field elevation.

• Journal of Welding and Joining
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• v.5 no.1
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• pp.64-72
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• 1987

The Weldability of gavannealed steel using spot welder has been studied. The Results obtained are a follows; 1) Welds size and strength were increased depending on the welding time and welding current. But, the increasing rate has been decreased. 2) Deposited zinc has affected on the wear of welding tips and growth of welds but has not affected the weld's structures. 3) On shear testing of the specimen, button fracture has been observed and the value was approximately 540Kg (welds dia. approximately .phi.4mm)

• Journal of Welding and Joining
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• v.31 no.6
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• pp.27-31
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• 2013

Magnesium alloy is used on parts of aircraft and electronic equipment because of the highest specific strength among the common metal materials. Recently, studies about appling magnesium alloy sheet to automotive bodies are on the increase rapidly. For application to automotive bodies, researches about characteristics of resistance spot welding of magnesium alloy sheet are essential. Magnesium alloy has low boiling point, so getting sound bead shape is difficult when appling varies welding processes. Resistance spot welding is also particular about setting optimum welding conditions because of spatter generation, pores and cracks occurrence in nugget. And life of electrodes is very short because of alloying with copper that main material of electrodes. This requires frequent dressing and replacement of electrodes and decrease in productivity of resistance spot welding on magnesium alloy. Therefore in this study, for effective analysis of changes in tensile shear load and nugget size during electrode life test, evaluate detail characteristics of resistance spot welding on magnesium alloy sheet using dome type electrode.

• Transactions of the Korean Society of Automotive Engineers
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• v.22 no.4
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• pp.30-35
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• 2014

According to weight lightning trend of automobile body, necessity of resistance spot welding (RSW) of Mg alloy on automobile industry is increasing rapidly because of the highest specific strength among commercial metals. Mg alloy has low electric resistance and high thermal conductivity like as Al alloy, so that needs applying high current in short time when welding. Thick oxide film of Mg alloy pollutes the electrodes and makes partial current carrying paths when on welding. Partial current carrying paths signify excessive concentration of current. There can initiate expulsion easily and reduces electrode life rapidly. Generating partial current carrying paths and expulsions are influenced by shapes of electrode. Therefore, electrode life also influenced by shape. In this study, we perform life test of RSW electrode of radius type. Measure tensile shear load and nugget size every spot alternately. As a result, radius type electrode can extend life over twice as dome type electrode.

• Journal of Welding and Joining
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• v.32 no.6
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• pp.1-7
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• 2014

For the resistance spot welds of CR1180 and GA1180 TRIP steels, the weld quality evaluation method using the digitalized X-ray transmission imaging apparatus was investigated in comparison with the crosssectional examination method. In the case of the resistance spot welding of CR1180, three circular regions, such as WZ(white zone), GZ(grey zone) and DZ(dark zone), appeared on X-ray image and they corresponded to the diameters of indentation mark, nugget and corona bond, respectively. The variation of X-ray transmission thickness due to the thickness variation of the weld seemed to be mainly responsible for the formation of those contrasts. The X-ray image contrast formed from the variation of transmission thickness at the outer border line of DZ could also enable the inspections of the notch shape, nonuniformity of the welding pressure and spatter from its sharpness, concentricity and the normal straight line, respectively. The X-ray image of the resistance spot weld of galvannealed GA1180 TRIP steel was very similar to that of CR1180 TRIP steel except the crown shaped outer border line of DZ which was considered to be due to the melting behavior of zinc having the boiling temperature even lower than the melting temperature of steel.

• Journal of Welding and Joining
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• v.35 no.2
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• pp.52-57
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• 2017

In this study, resistance spot weldability of lightweight steel with high Al contents was evaluated using various electrode shapes. The six types of electrode shape were prepared with different electrode face diameter and radius. The tensile shear tests were carried out to investigate the failure behaviors. Also, the nugget size and hardness were measured and compared with various electrode shapes. The experimental results show that the acceptable weld current region for low density lightweight steel could be obtained with 10mm electrode face diameter and 76mm electrode face radius.

• Journal of Ocean Engineering and Technology
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• v.11 no.3
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• pp.20-28
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• 1997

The fatigue crack propagation properties and fatigue life of two kinds of Al body panel for automobile were examined experimentally by using the plate specimen and the single spot welding specimen. The fatigue limit of spot welding specimens was lower than that of a plate specimen. The fatigue limit was similar in two kinds of spot welding specimen. The shape and size of crack propagation were observed and measured on beach mark of fracture surface. The crack propagation of surface crack specimen showed almost same tendency to that of a thick plate as almost semi-elliptical. In spot welding specimen, the fatigue crack occurred in inside surface of nugget area was almost semi-elliptical. The crack growth rate can be explained using equation of stress intensity factors.

• Korean Journal of Metals and Materials
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• v.49 no.1
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• pp.64-72
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• 2011

The spot weldability of dissimilar metal joints between austenitic stainless steels (STS316)/IF steels and ferritic stainless steels (STS430)/IF steels was investigated. This study was aimed to determine the spot welding parameters for a dissimilar metal joint and to evaluate the dissimilar metal joint's weldability, including its welding nugget shape, tensile-shear strength, hardness, and microstructure. The comparison of these results was described in terms of fracture behavior. Compared with the weld lobe of similar metal joints, dissimilar metal joints (STS430/IF) had reduced weld current range. However, the weld lobe of STS316/IF steel joint showed increased weld current range. This is because the dilution of chemical composition in the molten weld pool suppressed the heat input being caused by Joule heat with current flow through the samples. The microstructure of the fusion zone was fully martensite and mixture of ferrite and martensite for austenitic stainless steel/IF steel and ferritic stainless steel/IF steel combination, respectively. The experimental results showed that the shape of nugget was asymmetric, in which the fusion zone of the austenitic and ferritic stainless steel sheet was larger due to the higher bulk-resistance. The predicted microstructure by using the Schaeffler diagram was well matched with experimental results. After peel test, the fracture was initiated from heat affected zone of ferritic stainless steel sheet side, however the final fracture was propagated into the IF steel sheet side due to its lower strength.

• Korean Journal of Metals and Materials
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• v.50 no.2
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• pp.136-145
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• 2012

Resistance spot welding of TRIP780 steels was investigated to enhance understanding of weld fracture mode after tensile shear testing (TST) and L-shape tensile testing (LTT). The main failure mode for spot welds of TRIP780 steels was partial interfacial fracture (PIF). Although PIF does not satisfy the minimum button diameter (4${\surd}$t) for acceptable welds, it shows enough load carrying capacity of resistance spot welds for advanced high strength steels. In the analysis of displacement controlled L-shape tensile test results, cracks initiated at the notch of the faying surface and propagated through the interface of weldments, and finally, cracks change path into the sheet thickness direction. Use of the ductility ratio and CE analysis suggested that the occurrence of PIF is closely related to high hardness and brittle welds, which are caused by fast cooling rates and high chemical compositions of TRIP steels. Analysis of the hold time and weld time in a welding schedule demonstrated that careful control of the cooling rate and the size of a weld nugget and the HAZ zone can reduce the occurrence of PIF, which leads to sound welds with button fractures (BFs).